The present invention relates to a conversion unit for converting a firearm such as a military assault rifle to a submachine gun. The exemplary conversion unit is described herein with respect to use with a lower receiver for assault rifles in the AR family including the 5.56 mm M16 and 7.62 AR-10 and AR-308, and converting such rifles to submachine guns capable of firing pistol-sized cartridges such as 9 mm, or other cartridges that are shorter than cartridges for which the rifle was designed.
Magazines for 9 mm or similar cartridges are typically shorter front-to-back and of a different width than customary AR magazines for 5.56 and 7.62 cartridges. Prior art adapting the AR style lower receiver to receive and retain such magazines include U.S. Pat. No. 6,070,352 and FIG. 3 therein.
Submachine guns are usually more compact than AR style rifles. Shorter weapons can be more effective in tight spaces such as rooms, stairwells, and other close combat environments where increased rate of fire compensates for less accuracy. Shorter weapons, such as Uzi or HK MP5 submachine guns, are often preferred in these situations, whereas the AR style rifles typically have longer barrel, and a butt stock to accommodate a recoil spring and buffer are less advantageous.
Military weapons must work reliably under a variety of conditions, often adverse. Sometimes only substandard ammunition is available. Variations in combustion gas pressures resulting from substandard ammunition can affect gas operating systems. Tight tolerances for moving parts, such as the bolt within the upper receiver can lead to jamming or other malfunctions. Thus the tolerances in automatic weapons are often generous enough to provide for proper irrigation of debris. However, generous tolerances permit off-axis movement of parts, such as the bolt, increasing wear of any bearing surfaces. Such wear leads to even greater tolerances and further wear. Converting an AR, which typically has an aluminum upper receiver containing a steel bolt, to a submachine with a higher cyclic rate exacerbates the problem. A higher cyclic rate creates more wear and more debris which in turn increases off-axis movement of the bolt within the upper receiver housing. Such movement at a high cyclic rate abrades the softer aluminum, or other softer materials, of the upper receiver housing.
Preferably the conversion unit installs on an AR lower receiver with no tools or extra parts such as a magazine well adaptor.
A conversion unit should be simple and with few variables and few components to enable an operator familiar with an AR to convert between assault rifle and submachine gun with speed and efficiency, and effectively operate both weapons.